This invention relates to phase control systems and more particularly to an improved phase locked loop circuit.
In order to maintain a phase locked relationship between controlled (C) and reference (R) signals, the phase control circuit compares phases of these signals and adjusts the controlled frequency relative to the reference frequency. In accordance with prior practice, if the controlled frequency is too high, the C-signal is held or delayed (no phase change) until occurrence of the R-signal or pulse. If the controlled frequency is too low, the R-signal is held (no phase change) until occurrence of the C-signal. A disadvantage of the latter practice of holding the reference signal is the necessity of interrupting operation of the source R-signals. This prevents the use of one reference source for several phase locked loops. Furthermore, this technique may complicate the design of sample-hold type phase detectors because the saw-tooth ramp voltage generator in the phase detector must be held at the upper limit of the operating range whenever a sample pulse (C-signal) does not occur. In addition, when a sample-hold type phase detector is used, if the controlled oscillator frequency becomes too low, oscillation may stop altogether with the output of the phase detector at a value which causes low oscillator frequencies so that the system at worst becomes locked in this condition or at best has a slower response. These disadvantages are overcome in the phase locked loop embodying this invention.
Another problem to which this invention is addressed is negative effect of circuit transients which occur when a pulse in one train passes a pulse in another train as when two normally synchronized pulse trains drift relative to each other. Avoidance of such transients is highly desirable in the operation of phase locked loops since transients tend to slow down or prevent the reacquisition of the lock condition.